Prediction of boridenes as high-performance anodes for alkaline metal and alkaline Earth metal ion batteries.

We conducted a comprehensive density functional theory investigation using the r 2 SCAN-rVV10 functional on the structural stability and electrochemical properties of boridenes for their use as anode materials in rechargeable alkaline (earth) metal-ion batteries (Li + , Na + , K + , Mg 2+ and Ca 2+ ). According to first-principles molecular dynamics simulations and reaction thermodynamic calculations, Mo 4/3 B 2 (OH) 2 and Mo 4/3 B 2 F 2 are unstable in the presence of alkaline (earth) metal ions due to the surface-conversion reactions between the surface terminations and adsorbates. Meanwhile, the bare Mo 4/3 B 2 and Mo 4/3 B 2 O 2 monolayers not only can accommodate alkaline (earth) metal ions, but also form stable multi-layer adsorption structures for most of the studied metal ions (Li + , Na + , K + , Mg 2+ and Ca 2+ ). The predicted gravimetric capacities of the bare Mo 4/3 B 2 monolayer (Mo 4/3 B 2 O 2 ) are 625.9 mA h g -1 (357.3 mA h g -1 ), 247.20 mA h g -1 (392.1 mA h g -1 ), 101.8 mA h g -1 (206.4 mA h g -1 ), 667.0 mA h g -1 , and 413.0 mA h g -1 (485.4 mA h g -1 ) for Li + , Na + , K + , Mg 2+ and Ca 2+ ions, respectively. The bare Mo 4/3 B 2 exhibits lower onset charging open circuit voltages for alkaline (earth) metal ions than that of Mo 4/3 B 2 O 2 . The diffusivities of the metal ions were revealed to be high on the boridene monolayer especially for the outer fully stable adsorption layers, where the migration energy barriers were found to be less than 0.10 eV. Similar to that of MXenes, the negative electron cloud (NEC) also plays a vital role in stabilizing the observed multi-layer adsorption structures for various metal ions on either the bare Mo 4/3 B 2 or Mo 4/3 B 2 O 2 monolayer.